Means to overcome differential thermal expansion effects in bimetallic splined shaft assemblies

ABSTRACT

A steel shaft having peripheral lands is broached by a splined carbide blank to form a broached assembly having interfaces between the lateral flanks of the lands and lateral flanks of the splines. The inner and outer peripheral portions of the steel shaft are spaced from the carbide blank to prevent radial stresses from being produced during broaching and to accommodate thermal expansions in the broached assembly. Circumferential chip grooves are provided in that portion of the shaft which is to be broached by the carbide blank to prevent continuous chips from forming during the broaching process which could seize on the cutting face of the carbide blank. The carbide blank of the broached assembly resists displacements relative to the steel shaft. The interfaces may be angularly disposed to alleviate large circumferential thermal stresses.

United States Patent John A. Bennett Peterborough, England Oct. 27, 1969 May 4, 11971 Chandler Evans, Inc. West Hartford, Conn.

Inventor Appl. No. Filed Patented Assignee MEANS TO OVERCOME DIFFERENTIAL THERMAL EXPANSION EFFECTS IN BIMETALLIC SPLINED SI-IAFT ASSEMBLIES [56] References Cited UNITED STATES PATENTS 3,381,548 '5/1968 Wolkenstein 74/801 Primary Examiner-Leonard H. Gerin AttorneyRadford W. Luther portions of the steel shaft are spaced from the carbide blank to prevent radial stresses from being produced during breaching and to accommodate thermal expansions in the broached assembly. Circumferential chip grooves are provided in ,thatpor:

tion of the shaft which is to be broached by the carbide blank to prevent continuous chips from forming during the breaching process which could seize on the cutting face of the I v carbide blank. The carbide blank of the broached assembly resists displacements relative to the steel shaft. The interfaces; may be angularly disposed to alleviate large circumferential thermal stresses.

4 Claims, 4 Drawing Figs.

US. Cl. 74/466, 29/159.2, 287/5-3(SS) Int. Cl. ..Fl6h 55/06, B2lb 53/28, F16d 1/06 Field of Search 74/446; 29/1592; 287/53 (SS) MEANS TO OVERCOME DIFFERENTIAL THERMAL EXPANSION EFFECTS IN BIMETALLIC SPLINED SHAFT ASSEMBLIES BACKGROUND or THE iNvENTioN The invention relates generally to metal working and particularly to a method of broaching together two dissimilar materials. The invention further relates to assemblies of two dissimilar metals which incorporate features to overcome the stresses induced by thermal expansions. The invention also relates to gear and journal assemblies in gear pumps.

When the inlet port'of a gear pump containing an intermeshing pair of 'gears is'connected to a'source of fluid, rotation of gears in one direction'will'cause theliquid to be drawn through the inlet port into the housing and carried around in pockets formed between the adjacent teeth of both gears and the housing and thence discharged to the outlet port. The liquid is drawniinto the housing due to the increasing free space between the housing adjacent the inlet port as the teeth of the two gears move out of engagement, and is discharged due to the decreasing free space within the housing adjacent the outlet port as the teeth move into engagement, inlet and outlet ports being substantially isolated from one another by small clearances between the teeth and the housing.

The volumetric efficiency of such a gear pump depends upon the closeness of the fit between the tips of the gear teeth and the enclosing housing. When such a gear pump is utilized to pump a fluid containing a highly abrasive contaminant, the clearances between the tips of the gearteeth and. the surrounding housing rapidly become enlarged and the pump volumetric efficiency falls off. One well-known method of providing a solution to this problem of rapid high wear is to insert a wear block in'the pump housing adjacent the pump outlet. This wear block is urged by discharge pressure into engagement with the periphery of the gear teeth adjacent the pump discharge area.

It has beencustomary to manufacture the wear block and the gears of an extremely hard wear-resistant material, such as tungsten carbide, so that the peripheral engagement of the gear teeth and the mating arcuate wiping surfaces of the wear block present a surface that can withstand the highly abrasive action of the contaminant fluid. However, the use of tungsten carbide gears creates certain problems, prominent among which is the manner of attachment of the gear to the shaft or journal. A one-piece carbide gear and shaft assembly would not be practical due to the bending stressesimposed upon the shaft by the pressure load on the gear and the necessary splines required in the shaft for attachment to a drive means. As carbide is relatively brittle, the shaft and/or splines are likely to fail when subjected to repeated loading. Another reason for not utilizing such an assembly is that prohibitive bearing wear may result from the use of a carbide shaft.

It would, of course, be possible to braze the carbide gear upon a steel shaft but it has been found that this method of attachment is unsatisfactory. All metals when heated, expand, and, on cooling, return to their original size. Since tungsten carbide has a coefficient of thermal expansion approximately half that of steel in a range from room temperature to about l,250 F., the expansion of carbide during the brazing operation is only about half that of the steel shaft and it is this phenomenon which creates most of the problems in brazing. When cooling from the brazing temperature the steel shaft tends to contract more than the carbide and in doing so creates high stresses in the carbide. The'brazed joint between the carbide gear and steel shaft has also been found to be prone to fatigue failure when subjected to repeated torsional stresses. I

A method was investigated to solve the above problem of attaching a carbide gear to a steel shaft, which involved broaching a carbide gear having internal spline serrations upon a steel shaft to shear mating spline serrations thereon for locking the two component parts together. It was found that this method caused internal radial stresses to develop in the carbide which were manifestedby'failure of the carbide blank as gearteeth were cutt'her eonl' SUMMARY OF TI-IE INVENTION The invention provides a rigid assembly of dissimilar materials which interface with each other in such aimanner that substantial thermal expansions are accommodated within the assembly without engendering deleterious stressestherein. The interface between thev components is aligned with the "direction of the resultant thermal expansion 8 thereat so that g led interface will be automatically obtained.

The invention provides a method by which one structure may be broached upon another structure so that a firm engagement is established therebetween without rendering the broached assembly susceptible to failure from thermal stresses during exposure to pronounced temperature changes.

It is accordingly an object of the invention toiprovide a method of broaching together two dissimilar materials so that thermal stresses in the broached assembly are minimized.

Another object is to provide an assembly of two dissimilar materials which is resistant to failure induced by thermal expansions.

Yet another object isto providea method for broaching .a carbide blank upon a steel member to produce an assembly which accommodates thermal expansions without being subjected to deleterious thermal stresses. r A further object is to provide an assemblyof two dissimilar components which interface with one another in a direction of the resultant thermal expansion thereat. 4

These and'other objects and advantageslwill become apparent from the following detailed description and accompanying drawing.

' BRIEFDESCRIPTION OF THE DRAWINGS I FIG. 1 is a longitudinal sectional view of a carbidegear and steel shaft assembly. v

FIG. 2 is a fragmentary longitudinal view of the steel shaft per se.

FIG. 3 is a transverse sectional view of the assembly of FIG. ltaken alongtheline3-3. .5

FIG. 4 is a transverse view-of another. steel shaft and carbide blank assembly broached in accordance with theinvention which incorporates a feature to accommodate large differential thermal expansions. I

DETAILEI) DESCRIPTION incorporated in a gear pump. Shaft 10 has a plurality of longitudinally extending lands 14 on the periphery thereof. Each land 14 includes an outer peripheral surface 16 and lateral flanks l8 and 20. The inner peripheral surface of shaft 10 is defined by surfaces 22 which extend between the opposite flanks of neighboring lands.

Blank 8 comprises a plurality of longitudinally extending splines 25 having lateral flanks 26 and 28. The interior peripheral surfaces'30 of splines 25 extend' between flanks 26 and 28. Longitudinally extending recesses 24 are formed between neighboring splines 25, the recesses partially receiving the lands of shaft 10 after the broaching process has been accomplished. As shown in FIG. 3 after broa'shing of the blank 8 uponthe shaft 10 the outer peripheral surfaces of lands',14 and the inner peripheral surfaces 22 of shaft 10 are spaced from the carbide blank 8. This geometrical relationship not only prevents radial stresses from being produced in the blank during the breaching process but also allows for a limited differential thermal expansion of the carbide blank and steel shaft.

Turning now to FIG. 2 wherein the detailed land arrangement is shown it will be observed that steel shaft 10 comprises an enlarged portion generally designated at 32. Enlarged portion 32 is tapered at both ends to form an angle of 150 with the surface of the shaft to facilitate the machining thereof and the breaching of the blank thereon. Lands 14 each comprise a plurality of circ'umferentially extending chip grooves 34 which are machined thereon to prevent the formation of long continuous chips during the breaching process.

Prior to breaching, splines 25 are cut into blank 8 by a conventional Eloxing process. The oppositely disposed flanks of neighboring splines are generally formed parallel to a radius which passes through the center of the recess between the splines, although the flank angle can be varied if desired when large temperature variations are anticipated, as is described more fully hereinafter. Lands 14 are then machined on steel shaft 10 such that flanks 18 and 20 are generally parallel to a radius passing through the center of the land. The transverse width of each of the lands 14 is initially slightly larger than is the transverse width of each of the recesses 24 to permit the respective lateral flanks of the recesses to breach the lateral flanks of the lands. This width differential should be of the order of a thousandth of an inch.

Blank 8 is secured in a suitable work holder and shaft is placed in axial alignment with blank 8 such that the lateral flanks of each land respectively axially overlap the corresponding lateral flanks of each recess and the inner and outer peripheral surfaces of the shaft are radially spaced from the blank. Axial movement of the shaft relative to the blank 'then causes the flanks of the recesses to respectively cut into the lands. As the shaft is axially moved into the blank by a hydraulic press or'other suitable drive member flanks 26 and 28 cut into the sides of land 14. As the shaft advances into the blank it will be noted that the only contact between the shaft and the blank is along the abutting flanks thereof by virtue of the spacing provided between surfaces 16 and 22 and the blank and therefore the radial stresses are not produced in the blank during the breaching process. After the breaching process has been completed and the blank substantially envelopes enlarged portion 32 of shaft 8, gear teeth are formed on the blank. Appropriate finishing operations are then performed upon the broached assembly to render the assembly suitable for incorporation in a gear pump. The steel shaft is preferably made of a soft steel such as nitralloy, and if such is the case it may be desirable to finish the surface thereof by a nitriding process, this process being well known to those skilled in the art.

It will be noted that the broached assembly of FIGS. 1 and 2 is adapted to be used only in pumps which are not subjected to extreme variations in temperature (i.e., over 200 F.). When subjected to an increase in temperature, surfaces 22 and 16 of shaft will be displaced closer to the blank 8 during this temperature change. During temperature increases lands 14 will, of course, produce circumferential stresses in the blank 8 by virtue of the larger coefficient of expansion of steel. However, these circumferential stresses should not induce failure of the blank 8 unless the temperature change is excessive.

If the application dictates that the shaft assembly should be able to accommodate significant temperature changes without becoming susceptible to failure because of the stresses induced in the blank, a shaft assembly as illustrated in FIG. 4 should be employed. The shaft assembly shown in FIG. 4 is the same as that of FIG. 1 with the exception of the angular relationship of the flanks of the lands and recesses to the shaft. Further, the assembly of FIG. 4 is broached by the same method described with reference to FIG. I.

Referring to FIG. 4, a shaft assembly generally indicated at 40 has a hollow carbide blank 42 broached upon a steel shaft 44. Blank 42 comprises a plurality of splines 46 which define recesses 47 and have flanks 48 and 50. The flanks of the splines form predetermined angles with the radial direction 2 and are cut to the desired flank angle by Eloxing. Shaft 44 has a plurality of lands 52 which have flanks 54 and 56 machined to the same angles as flanks 48 and 50. The outer and inner peripheral surfaces 58 and 60 of shaft 44 are spaced from the carbide blank as in the previously-described embodiment. Lands 52 each have a circumferential width 28 that is measured along an arc midway of the radial depth of the lands. This are is a radial distance R from the center of shaft 44. When the assembly of FIG. 4 is exposed to a temperature change the difference in the expansion coefficients of steel and carbide results in a differential expansion of the two components of the assembly.

The total resultant expansion E, of points 62, which are defined by the intersections of the midway arcs along which the width 28 is measured and flanks 54 and 56 of lands 52, may be represented by a radial expansion E and a circumferential expansion E These expansions may be computed by' the following equations wherein:

AT Overall Temperature Change;

R Radius of the point 62 on the interface between abutting flanks 50 and 56;

B The circumferential half width of a land of shaft 44 measured along an arc midway of the radial depth of the land;

6 The angle of the interface between the carbide blank and steel shaft from the radial direction of point 62;

A, Expansion Ceefiicient, Steel;

A, Expansion Coefficient, Carbide.

Referring to FIG. 4 and assuming a uniform temperature change AT has been occasioned throughout the assembly, it is obvious that:

Bearing in mind that equations (1) and (2) represent the components of the expansion of interface point 62, a resultant expansion E, lying in the direction of the interface would require that:

But, in order for equation (3) to be valid the tangent of the angle 0 must be related to the component expansions as follows:

TANB=EJE (4) Substituting the values of E and E from respective equations (1) and 2), it is apparent that equation (4) may be rewritten as follows:

TANO=B/R (5) If this breaching is performed with both carbide and steel raised to the maximum temperature expected in operation and then allowed to cool, the assembly will automatically be capable of operating at that particular temperature without encountering unwanted interference.

It will be observed that as B, the midpoint width of a land increases, 0 correspondingly increases. If the selected values of B are small with respect to a given R significant stresses will be avoided along the entire interface between flanks 50 and 56. It will further be observed that as the assembly 1 undergoes a temperature change flanks 50 and 56 will slide along one another. Thus in this arrangement an increase in temperature results in sliding between the abutting flanks and'movement of surfaces 58 and 60 towards blank 42. i

It will be understood that the arrangement illustrated in FIG. 4 is not limited to any particular process of manufacture of materials but would find utility irrespective of the method of securing one component upon the other. Other modifications in the disclosed process and arrangements are contemplated within the scope of the invention.

lclaim:

1. In a shaft assembly, the combination comprising:

a structure having a hollow central portion which includes a recess bounded by lateral flanks;

a shaft having a peripheral land bounded by lateral flanks and an outer peripheral surface mounted within the central portion of the structure, the recess receiving the land such that the flanks of the recess respectively abut the flanks of the land to form interfaces therebetween and the inner and outer peripheral surfaces of the shaft are spaced from the structure, the interfaces forming angles with the respective radial directions which have tangents that substantially equal B/R, wherein B is the half width of way of the radial depth of the land; and whereby temperature changes will cause the abutting flanks to slide over one another, thereby preventing deleterious thermal stresses in the assembly. 2. The combination, as defined in claim 1, wherein the structure is made of a material harder than that of the shaft.

3. The combination, as defined in claim 2, wherein the structure'is made of carbide and the shaft is made of steel.

4. The combination, as defined in claim 1, wherein the the land midway of the radial depth thereof and R is the 10 structure includes aplurality ofteeth' radial distance from the center of the shaft to points mid- 

1. In a shaft assembly, the combination comprising: a structure having a hollow central portion which includes a recess bounded by lateral flanks; a shaft having a peripheral land bounded by lateral flanks and an outer peripheral surface mounted within the central portion of the structure, the recess receiving the land such that the flanks of the recess respectively abut the flanks of the land to form interfaces therebetween and the inner and outer peripheral surfaces of the shaft are spaced from the structure, the interfaces forming angles with the respective radial directions which have tangents that substantially equal B/R, wherein B is the half width of the land midway of the radial depth thereof and R is the radial distance from the center of the shaft to points midway of the radial depth of the land; and whereby temperature changes will cause the abutting flanks to slide over one another, thereby preventing deleterious thermal stresses in the assembly.
 2. The combination, as defined in claim 1, wherein the structure is made of a material harder than that of the shaft.
 3. The combination, as defined in claim 2, wherein the structure is made of carbide and the shaft is made of steel.
 4. The combination, as defined in claim 1, wherein the structure includes a plurality of teeth. 